This invention relates to electrical connectors and, in particular, to a snap connector for attaching an electroluminescent lamp to a printed circuit board.
Electroluminescent (EL) lamps are used in a variety of applications in which a lamp must be connected to a printed circuit board. (As used herein, "printed circuit board" is intended to include any substrate having one or more layers of conductive traces for interconnecting electronic devices, whether the substrate is rigid or flexible.) Portable communication devices use one or more EL lamps for backlighting displays, backlighting keypads, or for displaying information directly, e.g. as seven segment displays. Providing a low cost, reliable connection between an EL lamp and a printed circuit board has been a continuing problem.
An electroluminescent (EL) lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer may include a phosphor powder or there may be a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current.
The front electrode is typically a thin (.apprxeq.100 nm.), transparent layer of indium tin oxide (ITO) or indium oxide on a transparent substrate of polyester or polycarbonate material having a thickness of about 7.0 mils (0.178 mm.). The rear electrode is typically screen printed using an ink containing solvent, a polymer binder such as polyvinylidene fluoride (PVDF), polyester, vinyl, or epoxy, and conductive particles such as silver or carbon.
Reliably making an electrical connection to an EL lamp is difficult because the lamp materials are relatively frail. The ITO layer is easily damaged and the rear electrode overlies the active area of the lamp. Thus, additional steps are required for reinforcing the lamp and for providing contact areas that are substantially more rugged than the lamp itself. The rear electrode must extend over a portion of the transparent substrate and be isolated from the front electrode, typically by patterning the front electrode. The tab or tail thus formed for connection makes the lamp asymmetrical and reduces the number of lamps that can be obtained from a panel of a given area.
A solder connection to an EL lamp requires a crimp fitting for receiving the solder and requires care during soldering not to melt the lamp. One could attach a thin wire to each electrode of an EL lamp but the wires would not be rugged enough for insertion into a socket. Customers for EL lamps want to assemble the lamps into products as quickly as possible, typically by using a pin and socket connection. Customers also do not want to perform any steps other than making the actual connection between a lamp and a printed circuit board.
Mechanical connectors, such as pin connectors and crimp fittings, must be secured to an EL lamp without destroying the conductor to which contact is being made. The problem is compounded by the fact that the lamp must remain relatively flat, i.e. the connector cannot distort the lamp. Unfortunately, a mechanical connection that does not distort the lamp is not likely to make a reliable electrical connection and a reliable electrical connection crimps the lamp so tightly that the lamp is no longer flat, even if the connector is separated from the luminous portion of the lamp by a short distance, e.g. 6 mm. A mechanical connector provides the advantage of physically locating a lamp in addition to making electrical connection to the lamp.
An EL lamp is essentially a two dimensional device because the thickness of a lamp is so small relative to the length and width of the lamp. Pins and other connectors add considerably to the thickness of a lamp, making the lamp more difficult to handle, package, or store. It is highly desirable to retain the advantages of a mechanical connection without changing the two dimensional nature of a lamp.
In the prior art, mechanical connections are intended to be permanent; i.e. the connection is not intended to be made and broken repeatedly. A pin and socket connector often includes a one-way type of fitting in the socket that makes removing the pin quite difficult. Other connections are mechanically secured and then soldered for reliability. Thus, a semi-permanent, reliable connection is not available in the prior art.
A pin and socket assembly that is then soldered is probably the least desirable connector from a manufacturing viewpoint. Special hardware is required for an EL lamp and for the device to be connected to the lamp, i.e. parts and pre-assembly are required. The lamp is attached in another operation and then soldered in a post-assembly operation, often followed by a cleaning step. Each step is a potential source of defect and each step has a cost associated with it. It is desired to minimize the number of steps required to manufacture a product incorporating an EL lamp.
U.S. Pat. No. 4,617,195 (Mental) discloses an EL lamp having leads extending from one side of the lamp and including front and rear electrodes patterned to avoid shorting. A pin connector is also disclosed. U.S. Pat. No. 5,045,755 (Appelberg) discloses an EL lamp having aluminum deposited on a portion of the substrate for making electrical contact to the lamp using spring contacts. U.S. Pat. No. 5,266,865 (Haizumi et al.) discloses attaching wire leads to an EL lamp and using staples or rivets for making connections to an EL lamp.
There is a fitting, known in the art as a surface mount test point, that is soldered to a printed circuit board for providing a temporary connection to a trace on the printed circuit board for testing or monitoring the operation of a circuit. The fitting includes a base and a ball connected to the base by a neck. A test lead is terminated in a suitable socket and the socket is connected to the ball to provide a rotatable, secure connection reminiscent of a trailer hitch or a grease fitting. In a commercially available test point, the ball has a diameter of 1 mm, the neck has a diameter of 0.5 mm, and the base has a diameter of 1.5 mm.
In view of the foregoing, it is therefore an object of the invention to provide a reliable, semi-permanent, electrical connection for an EL lamp.
Another object of the invention is to provide a connection that does not require special tools for attaching or removing an EL lamp.
A further object of the invention is to provide a snap-in connection for an EL lamp in which the lamp itself is part of the connector.
Another object of the invention is to provide an electrical connection that can be located anywhere in an EL lamp.
A further object of the invention is to provide a connection that requires no additional hardware attached to an EL lamp.
Another object of the invention is to provide an electrical connection for an EL lamp that lends itself to automated assembly.
A further object of the invention is to provide an electrical connection for an EL lamp that requires no post-assembly operations.
Another object of the invention is to provide an electrical connection for an EL lamp that also secures the lamp in a fixed position.